Mass flow measurement, preferably for controlling chip feed to a digester

Abstract

A method and system are provided for controlling the feed of comminuted cellulosic fibrous material (e.g. wood chips) to a continuous digester utilizing a chips meter of variable speed. A quality of the chips related to mass flow--preferably density--is continuously determined, preferably while the material is in free fall, not immersed in liquid. The speed of the material meter is controlled in response to the quality determined to provide a target mass flow of material from the meter to the digester. A nuclear type density gauge may be utilized having a source and a detector mounted on opposite sides of a chip chute, and above the level of liquid in a chip chute so that density is determined during free fall of the chips, not immersed in liquid. A level sensor may also be associated with the chip chute with a shield between the nuclear source and the level sensor. The chip meter speed is controlled in the range of about 0-16 rpm.

What is claimed is: 
     
       1. A method of controlling the feed of comminuted cellulosic fibrous material to a continuous digester using a material meter of variable speed, comprising the steps of: (a) substantially continuously feeding comminuted cellulosic fibrous material from the material meter to the continuous digester;   (b) during the practice of step (a), substantially continuously determining the density of the material being fed, utilizing a chute having a level of liquid therein, by passing a stream of nuclear particles through the flow of material to determine its density, and detecting the relative amount of particles passing through the stream;   (c) substantially continuously controlling the speed of the material meter in response to step (b) to provide a target density of material fed from the meter to the continuous digester;   (d) sensing the level of liquid in the chute with a level sensor; and   (e) shielding the level sensor from the nuclear particles.   
     
     
       2. A method as recited in claim 1 comprising a further step of steaming the material during the practice of step (a). 
     
     
       3. A method as recited in claim 1 wherein step (c) is practiced to control the material meter speed so that it is in the range of 0-16 rpm. 
     
     
       4. A method as recited in claim 1 wherein step (b) is practiced by determining the density of the material while it is in free fall, not immersed in liquid. 
     
     
       5. A method as recited in claim 4 wherein the material is selected from the group consisting essentially of hardwood and softwood wood chips. 
     
     
       6. A method of controlling the feed of comminuted cellulosic fibrous material to a continuous digester using a material meter of variable speed, comprising the steps of: (a) substantially continuously feeding comminuted cellulosic fibrous material from the material meter to the continuous digester;   (b) during the practice of step (a), substantially continuously determining the density of the material being fed while it is in free fall, not immersed in liquid; and   (c) substantially continuously controlling the speed of the material meter in response to step (b) to provide a target density of material fed from the meter to the continuous digester.   
     
     
       7. A method as recited in claim 6 wherein step (b) is practiced utilizing a chute having a level of liquid therein, and comprising the further steps of sensing the level of liquid in the chute with a level sensor, and shielding the level sensor from the nuclear particles. 
     
     
       8. A method as recited in claim 7 wherein step (c) is practiced to control the material meter speed so that it is in the range of 0-16 rpm.